1. Field of the Invention
The present invention relates to a method and an apparatus for electronically sensing a fingerprint on a finger surface, and more particularly to a capacitive fingerprint sensor with an adjustable gain.
2. Related Art
Accurate and cost effective verification of personal identity is becoming increasingly important. Verification of personal identity can be used to prevent calling card, prevent credit card fraud, deter theft and deter misuse of portable products, such as cellular phones and laptop computers, and ensure security in electronic commerce. Many methods have been proposed for electronic identification of individuals including: passwords, hardware tokens such as credit cards and ATM cards, and even portable encryption devices that combine the ideas of password and token. All of these have the problem that it is difficult or impossible to ensure that the token is in the possession of its rightful owner.
Biometric techniques rely on verifying identity by identifying a unique feature of the individual""s body, such as voice, fingerprint, hand print, signature, and retina pattern. These techniques have the advantage that they move with the individual and are theoretically capable of great accuracy. However, they suffer from the drawback that in many cases, acquiring the data requires complex equipment and complex interactions with the user. Comparison of the acquired biometric data with a database of biometric data can also be quite computationally intensive, and can consequently require tremendous computational resources. Furthermore, biometric techniques such as signatures and voice recognition are subject to relatively high error rates.
Of all presently-used biometric identification techniques, fingerprints are perhaps the most appealing. Fingerprints have been accepted for 75 years as a legal means for verifying identity xe2x80x9cbeyond all reasonable doubt,xe2x80x9d and acquiring a fingerprint requires little specific behavior by the user. Considerable research has gone into the task of extracting fingerprint features and performing database comparisons. Existing technology allows the relevant features of a fingerprint to be represented in as little as 10 bytes of data, with recognition in less than 1 second, and with false acceptance and false rejection rates of 0.01%. Furthermore, the computer hardware required for recording and comparing fingerprint data can be centralized and accessed through a telecommunications network thereby allowing costs to be amortized across many transactions.
The main barrier to the widespread use of fingerprint recognition is fingerprint acquisition. There must be an acquisition device at every identification site. Existing techniques for fingerprint acquisition rely on optically imaging the fingerprint onto a light sensitive detector such as a CCD, to obtain an electronic image of the fingerprint. This approach has a number of problems such as high cost, great complexity, large size, poor image quality, and susceptibility of the optical system to misalignment and breakage. Optical devices are also susceptible to xe2x80x9cspoofing,xe2x80x9d wherein a simulacrum of the fingerprint or even a photocopy of a fingerprint image is used to fool the optical sensor.
Another method for sensing fingerprints is by using a capacitive sensor, such as the sensor disclosed in U.S. Pat. No. 5,325,442, entitled, xe2x80x9cFingerprint Sensing Device and Recognition System Having Predetermined Electrode Activation,xe2x80x9d by inventor Alan G. Knapp (the xe2x80x9cKnapp patentxe2x80x9d). The Knapp patent discloses a fingerprint sensing device comprising a planar array of closely-space capacitive sense elements. When a finger is placed in close proximity to the sensing device, the capacitive sense elements measure a capacitance between the finger surface and single capacitor plate in each sense element. This is accomplished by sensing a drive current while driving a voltage onto the capacitor plate. The measured capacitance varies as a function of the distance between the capacitor plate and the finger surface. Thus, a capacitance measurement allows the distance between the capacitor plate and the finger surface to be determined. Distance measurements across the array of sense elements are combined to produce a representation of the pattern of ridges on the finger surface which comprise a fingerprint.
There are however a number of serious flaws with the design disclosed in the Knapp patent. (1) The Knapp invention operates by driving a charging current onto a column line which feeds into a sense electrode and measuring a charging current to determine a capacitance between the sense electrode and the finger surface. In such a configuration, because of length of the column line and the small size of the capacitance being measured, discriminating between the charging of the column line and the charging of the electrode plate is a difficult if not impossible task. (2) No provision is made in the design for variations in capacitance due to, for example, dry finger surfaces and temperature variations. (3) Additionally, the invention disclosed on the Knapp patent measures a charging current. This charging current is likely to vary widely over time as the electrode plate and the attached column line are charged. Hence, it is unlikely that a charge current measured at a particular instant in time will be an accurate indicator of capacitance.
What is needed is a capacitive fingerprint sensor with sufficient sensitivity to gather a fingerprint from a finger surface, and with sufficient adjustability to deal with variations in capacitance due to changes in moisture and temperature characteristics of the finger surface.
The present invention provides a method and an apparatus for detecting a fingerprint from a finger surface. The apparatus includes a planar array of capacitive sense elements disposed on a substrate. It also includes an insulating and receiving surface disposed over the array of sense elements, which is adapted to receive a finger so that a sense element and a portion of the finger surface located thereabove create a measurable change in capacitance. The capacitance is measured by first precharging each sense element, and then using a known current source to remove a fixed amount of charge from each capacitor plate. After a fingerprint is acquired, the quality of the fingerprint is evaluated, and if necessary, a gain parameter for the sense elements is iteratively adjusted until a satisfactory fingerprint is acquired.
Thus, the present invention can be characterized as a method for detecting a fingerprint from a finger surface using an array of sense elements containing electrode plates. The method comprises the steps of: placing the finger surface over the array of sense elements, so that the electrode plates within the sense elements form one plate of the capacitor, the other plate being the finger surface; charging the electrode plates to a reference voltage; draining charge off the electrode plates at a determinable rate; and measuring voltages from respective electrode plates after a determinable time interval has elapsed; and finally forming the voltages into a representation of the fingerprint from the finger surface.
According to an aspect of the present invention, the method includes the step of converting voltages into a digital form to create a digital representation of the fingerprint.
One embodiment of the present invention may also be characterized as a method for detecting a fingerprint from a finger surface, the method using at least one sense element, the method comprising the steps of: placing the finger surface over at least one sense element; measuring electrical signals from the at least one sense element, the electrical signals indicative of an interaction between the at least one sense element and the finger surface; forming the electrical signals into a representation of the fingerprint from the finger surface; evaluating the quality of the representation of the fingerprint; if the representation is of poor quality, adjusting measurement parameters of the at least one sense element, and measuring the electrical signals again to form another representation of the fingerprint. According to one aspect of this embodiment, the step of measuring electrical signals from the at least one sense element measures a capacitance between the at least one sense element and the finger surface. According to another aspect of this embodiment, the step of measuring electrical signals measures a resistance between the at least one sense element and the finger surface. According to yet another aspect of this embodiment, the step of measuring electrical signals gathers a signal from an optical scan of the finger surface. According to another aspect of this embodiment, the step of adjusting measurement parameters includes adjusting a gain of the at least one sense element.
The present invention may also be characterized as a apparatus for detecting a fingerprint on a finger surface, comprising: an array of sense elements, sense elements in the array including, an electrode plate which forms a capacitor with the finger surface, and an amplifier, which amplifies the voltage on the electrode plate; an insulating and receiving surface disposed over the array of sense elements; driving circuitry coupled to sense elements in the array of sense elements for driving a voltage onto electrode plates in the sense elements; and sensing circuitry coupled to the sense elements for sensing a voltage on the electrode plates in the sense elements.
According to an aspect of the present invention, sense elements include a current source which drains current form the electrode plates at a predetermined rate.
According to another aspect of the present invention, the apparatus includes at least one voltage measurement circuit for measuring voltage on an electrode plate in a sense element in the array of sense elements.
According to another aspect of the present invention, the sensing circuitry includes a plurality of lines extending through the array of sense elements, the plurality of lines carrying voltage signals from sense elements to voltage measuring circuitry on the periphery of the array, so that the lines traverse at most half of the array before arriving at the periphery of the array.
According to another aspect of the present invention, the driving circuitry includes: a plurality of lines extending through the array of sense elements, the plurality of lines delivering driving current to the sense elements; and a plurality of drivers on the periphery of the array coupled to the plurality of lines, for supplying the drive current to the sense elements, such that a pair of drivers are attached to opposite ends of lines in the plurality of lines.
According to another aspect of the present invention, the apparatus includes an addressing mechanism for selectively accessing at least one sense element from the array of sense elements.
According to yet another aspect of the present invention, the array of sense elements is located in a module, the module including; at least one analog-to-digital converter coupled to the sensing circuitry; a digital signal processor coupled to the at least one analog-to-digital convertor, for performing computations on a representation of the fingerprint gathered from the array; and an interface circuit for communicating with devices outside of the module.
According to another aspect of the present invention, sense elements in the array include: a drive input, including a drive switch to selectively switch driving current onto the electrode plate; and a sense output, including a sense switch to selectively switch voltage from the electrode plate to the sense output.
According to another aspect of the invention, sense elements in the array of sense elements include a current source, which drains current from the electrode plate at a predetermined rate.
Because the new capacitive sensor reported here is manufactured using a standard semiconductor fabrication process, it is cheaper, smaller, and more robust than optical systems. The metal capacitor plates are fabricated from metal layers on the integrated circuit. The control circuits, required for capacitance measurement and readout, are also fabricated on the same integrated circuit. No special techniques are required. Although this method requires silicon area as big as the fingerprint, the sensor elements can be made quite large to achieve the desired resolution and, speed is not a crucial issue. Hence, a relatively old and inexpensive CMOS processes (such as 1.2 micron or 2 micron) can be used. Furthermore, usable images can be acquired even if there are dead sensor elements. Both these factors will increase yield.
The capacitive sensing method also produces higher quality images than optical techniques because it can detect intermediate levels between the ridges and valleys of the fingerprint. Furthermore, because the scheme relies on the fact that the finger forms a plate of the capacitor, the finger must be able to conduct electricity. This eliminates the possibility of xe2x80x9cspoofingxe2x80x9d the device with a plastic simulacrum or a photocopy. Even though a conducting simulacrum might be created, typically it would not produce an image sufficiently like the original to generate a false acceptance.
In one embodiment of the present invention, images from the sensors contain gray level information. In another embodiment, binary images of the fingerprint are gathered and the sensor includes thresholding circuitry incorporated into the readout circuit to generate a binary output from the sense elements. The threshold is set locally in specific regions of a chip to correct for variations across the chip.